The propulsion system of an aircraft must be capable of ingesting foreign objects without engine damage. The problem of foreign object ingestion has been solved in the past by merely increasing the strength of the engine components exposed to impact damage. However, strength can be equated with weight, which, in turn, compromises performance of the aircraft. Reconciliation of such seemingly divergent performance and safety requirements requires careful integration of the aircraft's propulsion system with airframe aerodynamics.
The basic model of air flow past an aircraft fuselage assumes that air viscosity acts over a relatively thin region termed the boundary layer. The boundary layer exists in several states, namely, laminar, turbulent, wake and the external stream. In the laminar state, flow is stratified. Farther aft, laminar flow transforms into a turbulent state which is eddying in character. Turbulent flow subsequently transforms into a wake wherein the direction of flow may actually reverse.
While careful aerodynamic design can cause the boundary layer, whether laminar or turbulent, to remain attached to the aft fuselage at least to a point beyond the engine air inlets, relatively heavy foreign objects, for example, birds, tend to flow rearwardly of the aircraft outside the cylindrical surface defined by the maximum cross section of the fuselage due to their large inertial forces.
Thus, placement of the engines so that their inlets are entirely within the area defined by the forward (passenger) section of the fuselage (i.e., so that they cannot be seen by an observer located forwardly of the aircraft on the extended axis of the fuselage) precludes foreign object ingestion. At the same time, careful aerodynamic design can limit the boundary layer thickness at the engine inlet plane to a finite value so that a reasonable gap between the inner boundary of the inlet and the fuselage will permit the boundary layer to pass between the inlet and the fuselage and permit the inlet to ingest only high velocity free stream air thereby maintaining the high efficiency of the installation.
It is to be noted, however, that any solution to the problem of foreign object ingestion must take into consideration the problem of side line noise induced by close proximity of airframe appendages to high density or velocity airflow.